Melting relationships in the system CaO-MgO-SiO_2-H_2O at 1 kilobar pressure

Abstract

Solid-liquid-vapor phase relationships in the system CaO-MgO-SiO_2-H_2O have been determined at 1 kilobar pressure using the quench technique in cold-seal pressure vessels. The phase fields intersected by the composition joins Ca(OH)_2-Mg_2SiO_4 and Ca(OH)_2-MgSiO_3 have been delineated in the temperature range 700–950°C. From these results and from previous work in the bounding systems, a schematic diagram for the phase relationships on the vapor-saturated liquidus surface has been prepared. A thermal divide exists on this surface, corresponding to the melting of mixtures on the join Ca_2SiO_4-MgO in the presence of H_2O vapor. Original liquids with compositions on the SiO_2 side of the divide yield anhydrous crystalline phases and vapor on fractional crystallization, whereas original liquids with compositions on the CaO side of this divide yield hydrated crystalline phases (calciochondrodite and portlandite) plus vapor. The estimated compositions of isobaric invariant liquids at 1 kilobar pressure for the reactions: portlandite + calciochondrite + periclase + vapor ⇌ liquid, at 725°C calciocondrite + periclase + vapor ⇌ dicalcium silicate + liquid, at 875°C, are Ca(OH)_2−92.7%, Ca_2SiO_4−4.3%, MgO−2.5%, H_2O−0.5%, and Ca(OH)_2−62.1%, Ca_2SiO_4−25.6%, MgO−11.3%, H_2O−1.0%. These results provide a basis for interpretation of the phase relationships in the more complex system CaO-MgO-SiO_2-CO_2-H_2O which have bearing on the petrogenetic relationships between kimberlites and carbonatites.

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